Operating cable modems in a low power mode

ABSTRACT

An apparatus may include a receiver configured to receive chunks of data on a downstream channel from a cable modem termination system. The receiver may be further configured to enter a low power state in which the chunks of data cannot be received. Wake up circuitry may be configured to monitor data in the downstream channel for a wake up signal when the receiver is in the low power state.

RELATED APPLICATION

This application is a continuation of Ser. No. 10/355,244, filed Jan.31, 2003, which claims the benefit of U.S. Provisional Application No.60/433,770, filed Dec. 17, 2002, under 35 U.S.C. §119(e), the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to power saving modes forelectrical devices, and more particularly, to power saving modes forcable modem systems.

2. Description of Related Art

In cable modem systems, a cable modem termination system (CMTS) at oneend of a cable network typically services multiple cable modems (CMs)connected to the cable network. CMs are generally installed locally atthe end-user's location, and communicate with the CMTS, which may beinstalled at a cable company's facility. Lately, there has been interestin operating CMs as Multimedia Terminal Adapters (MTAs) by running aVoice over IP (VoIP) application on the CMs. The voice service providedby such a CM/MTA may be considered a “primary line” service. In otherwords, the voice service provided by the CM/MTA may be the primary way auser connects to other people and/or emergency services via the publicswitched telephone network (PSTN).

In the event of power loss, it is desirable for the user of the CM/MTAstill to be able to originate calls, receive calls, be notified ofincoming calls (e.g., by ringing), and so on. During such power loss,the CM/MTA may still receive power from an internal power source oruninterruptible power supply (UPS). To avoid excessive drain on the UPS,and to draw lower average power in general, it may be desirable for theCM/MTA to operate in a low power mode (i.e., a “power saving” mode).

For example, it may be desirable for the CM/MTA to operate in a lowpower mode during periods of non-use, so that the long-term, averagepower consumption of the device is less than about 3 Watts. Further, lowpower operation of one CM should not adversely affect operation of otherCMs served by the same CMTS. The current Data over Cable ServiceInterface Specification (DOCSIS) protocols (e.g., DOCSIS 1.1, 2.0),however, contain no provision for operating CMs in a low power mode.

Therefore, there exists a need to reduce power consumption of CMs incable modem systems.

SUMMARY OF THE INVENTION

Systems consistent with the principles of the invention address this andother needs by providing a low power mode for components in CMs, whilemaintaining a mechanism for an associated CMTS to wake up a CM that isin the low power mode.

In accordance with one aspect of the invention as embodied and broadlydescribed herein, an apparatus may include a receiver configured toreceive chunks of data on a downstream channel from a cable modemtermination system. The receiver may be further configured to enter alow power state in which the chunks of data cannot be received. Wake upcircuitry may be configured to monitor data in the downstream channelfor a wake up signal when the receiver is in the low power state.

In another implementation consistent with principles of the invention, acable modem termination system may include a transmitter for sendingchunks of data to one or more cable modems on a downstream channel.Signaling circuitry may be configured to insert a wake up message intothe chunks of data on the downstream channel. The wake up message is forcausing a device to exit a low power mode.

In a further implementation consistent with principles of the invention,a method of low power operation in a cable modem system may includedeciding to enter a low power state and removing power from at least oneof a transmitter and a receiver. A separate downstream communicationchannel may be monitored for a wake up message. The method may alsoinclude supplying power to the at least one of the transmitter and thereceiver upon reception of the wake up message.

In still another implementation consistent with principles of theinvention, a system may include a multimedia terminal adapter and acable modem termination system. The multimedia terminal adapter mayinclude a receiver and wake up circuitry. The receiver may be configuredto receive chunks of data on a downstream channel and may be furtherconfigured to be placed into a low power mode. The wake up circuitry maybe configured to monitor data on the downstream channel for a wake upsignal when the receiver is in the low power mode. The cable modemtermination system may be operatively connected to the multimediaterminal adapter by the downstream channel and may include atransmitting portion and signaling circuitry. The transmitting portionmay be configured to send the chunks of data to the multimedia terminaladapter. The signaling circuitry may be configured to insert the wake upsignal into the chunks of data for causing the multimedia terminaladapter to exit the low power mode.

In yet another implementation consistent with principles of theinvention, a multimedia terminal adapter may include a receiverconfigured to receive chunks of data on a downstream channel from acable modem termination system. Control circuitry may be configured toselectively remove power from the receiver. Wake up circuitry may beconfigured to monitor the chunks of data on the downstream channel for awake up signal when the power has been removed from the receiver.

In a further implementation consistent with principles of the invention,a method of low power operation in a cable modem system may includereceiving a sleep message from a cable modem indicating the decision ofthe cable modem to enter a low power state. The method may also includestopping one or more timers that control removal of inactive cablemodems from an active list due to inactivity of the inactive cablemodems. The one or more timers may be stopped in response to the sleepmessage.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, explain the invention. In the drawings,

FIG. 1 is a diagram illustrating an exemplary system in which conceptsconsistent with aspects of the invention may be implemented;

FIG. 2 is a diagram of exemplary upstream/downstream communicationsbetween a CMTS and multiple cable modems in the system of FIG. 1according to an implementation consistent with the principles ofinvention;

FIG. 3 is a diagram of an exemplary CMTS and CMs according to animplementation consistent with the principles of invention;

FIG. 4 is a diagram of the receiver (Rx) of FIG. 3 according to animplementation consistent with the principles of the invention; and

FIG. 5 illustrates an exemplary process for entering and exiting a CMpower saving mode consistent with the principles of the invention.

DETAILED. DESCRIPTION

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers may be used indifferent drawings to identify the same or similar elements. Also, thefollowing detailed description does not limit the invention. Instead,the scope of the invention is defined by the appended claims andequivalents.

Systems and methods consistent with the principles of the invention mayuse respective, dedicated wake up and signaling circuitry in a CM and aCMTS to return the CM, whose receiver has been placed into a low powermode, to normal operation. The low power mode may also be referred to asa “sleep mode” or a “power saving mode” as will be understood by thoseskilled in the electronics art.

System Overview

FIG. 1 is a diagram illustrating an exemplary system 100 in whichconcepts consistent with aspects of the invention may be implemented.System 100 may include a number of end users 110A-110N (collectively“users 110”), a number of cable modems (CMs) 120A-120N (collectively“CMs 120”), a cable network 130, a cable modem termination system (CMTS)140, and a network 150.

End-users 110 may connect to the cable network 130 through CMs 120. TheCMs 120 typically are installed local to the respective end-users 110.End-users 110 may include a device or a network (e.g., LAN) of devicesthat may include a television, a computer, a telephone, or any othertype of equipment that can receive and/or send data via cable network130. A LAN at an end-user 110 may include one or more of a wired networkand a wireless network served by, for example, a switch, router, or acomputer's network interface.

Cable network 130 may include a coaxial or hybrid optical fiber/coaxial(HFC) cable network. Cable modems 120 may interconnect with the cablenetwork 130 via coaxial cable/optical fiber.

CMTS 140 may facilitate communications between cable modems 120 andnetwork 150. CMTS 140 may be maintained by a cable company at a facilityin relative proximity to end-users 110. CMTS 140 may include a number ofupstream (i.e., from the cable modem to the CMTS) channels anddownstream (i.e., from the CMTS to the cable modem) channels. Forexample, the group of CMs 120 may be served by 16 upstream channels andfour downstream channels. The downstream channels may be higherbandwidth channels than the upstream channels. CMs 120 may share bothupstream and downstream channels by time-multiplexing data on thesechannels. CMs 120 transmit data units, called bursts, to CMTS 140 duringpre-assigned time slots.

Network 150 may include one or more networks of any type, including aPublic Land Mobile Network (PLMN), Public Switched Telephone Network(PSTN), local area network (LAN), metropolitan area network (MAN), widearea network (WAN), the Internet, or an intranet.

It will be appreciated that the number of components illustrated in FIG.1 is provided for explanatory purposes only. A typical network mayinclude more or fewer components than are illustrated in FIG. 1.

Exemplary Downstream/Upstream Communication

FIG. 2 illustrates exemplary upstream and downstream communicationbetween a CMTS 140 and CMs 120 according to an implementation consistentwith the principles of the invention. As illustrated in FIG. 2, CMTS 140and CMs 120 interconnect via upstream 210 and downstream 220 channels ofcable network 130. Each upstream channel 210 and downstream channel 220may communicate via a different frequency. Each upstream channel 210 mayfurther include multiple “virtual” channels. Each virtual upstreamchannel may include a multiplexed timeslot of the upstream channelfrequency. Each virtual upstream channel may further be associated withdifferent transmission characteristics of CMs 120. Such differenttransmission characteristics may include a different channel profile,such as different TDM timeslot size, symbol rate, frequency, preamblepattern, and/or burst profile. The different burst profile may include adifferent modulation, pre-amble length, data block size (e.g.,Reed-Solomon block size), error correction (e.g., Reed-Solomon errorcorrection), scrambling or encryption, encoding (e.g., differentialencoding), maximum burst size, and/or guard time size.

The upstream channels 210 from cable modems 120 may, thus, includefrequency bandwidth divided into multiple channels, with each channelpossibly further time division multiplexed into multiple virtualupstream channels. CMTS 140 may transmit messages and chunks of data(e.g., bytes, packets, etc.) to each CM 120 on a downstream channel 220and may receive transmissions from each cable modem via an upstreamchannel 210.

Exemplary Cable Modem Termination System and Cable Modems

FIG. 3 is a diagram of an exemplary CMTS 140, a CM/MTA 310, and a CM 320according to an implementation consistent with the principles ofinvention. Also illustrated is telephone device 330 configured to sendvoice data to CMTS 140 via VoIP applications in CM/MTA 310. CM 320 maybe configured in some implementations as an MTA and connected to anothertelephone device, but for the purpose of explanation, CM 320 will betreated as a data CM.

CM/MTA 310 may include a transmitter (Tx) 312, a receiver (Rx) 314,interface circuitry 316, and wake-up circuitry 318. CM 320 may besimilarly configured, with or without interface circuitry 316 for MTAoperation. Elements 312, 314, 316, and 318 are logical blocks that mayreside in a single chip or on multiple chips.

Tx 312 may be configured to transmit data to CMTS 140 on upstreamchannel 210. Tx 312 may be controlled, for example by interfacecircuitry 316 or another controller, to send data to CMTS 140 only intime slots that were assigned by CMTS 140, as will be understood bythose skilled in the cable modem art. In one implementation consistentwith the principles of the invention, Tx 312 may be “powered down” tooperate in a low power mode by removing a voltage supply from Tx 312.Such removal of voltage may be controlled by interface circuitry 316 oranother controller in a manner known to those skilled in the circuitdesign arts. In an alternate implementation, Tx 312 may use sufficientlylow power to be considered “powered down” if it is instructed not totransmit data.

Rx 314 may be configured to receive data from CMTS 140 on downstreamchannel 220. One constraint when “powering down” Rx 314 to operate in alow power mode is that CM/MTA 310 should still retain communicationcapabilities (i.e., the ability to receive, for example, a “wake-up”message from CMTS 140). The structure of Rx 314 and its implication forpower saving modes will be discussed in greater detail with respect toFIG. 4.

FIG. 4 is a diagram of Rx 314 according to an implementation consistentwith the principles of the invention. Rx 314 may include an analogfilter 410, an analog-to-digital (A/D) converter 420, data trackingcircuitry 430, symbol extraction circuitry 440, and MPEG audio parsingcircuitry 450. Analog filter 410 and A/D converter 420 may be consideredanalog circuitry, while the remaining elements 430-450 may be considereddigital circuitry.

Analog filter 410 may extract information from a certainfrequency/channel of an input RF signal. The bandwidth and criticalfrequency of analog filter 410 may be programmable, so that it mayselect one of several available downstream frequencies/channels. A/Dconverter 420 may be configured to convert the filtered signal fromanalog filter 410 to a digital signal.

Data tracking circuitry 430 may be configured to separate the digitalsignal from A/D converter 430 into usable data for later circuitry. Forexample, if the data transmitted by CMTS 140 is in quadrature format,data tracking circuitry 430 may determine into which quadrant aparticular portion of the digital signal should fall. Symbol extractioncircuitry 440 may extract symbols or other units of information (e.g.,characters or words) from the data output by data tracking circuitry430. MPEG audio parsing circuitry 450 may be configured to produce audiosignals from the symbols output by symbol extraction circuitry 440.

In one implementation consistent with the principles of the invention,Rx 314 may enter a low power mode by turning off, or otherwise removingpower from, the digital components: data tracking circuitry 430, symbolextraction circuitry 440, and MPEG audio parsing circuitry 450. Such animplementation would result in approximately a 50% reduction in poweruse by Rx 314 during low power mode. In such an implementation, theanalog components (i.e., analog filter 410 and A/D converter 420) wouldremain powered, because Rx 314 should retain the capability to receive(and act upon) a wake-up signal from CMTS 140. In such animplementation, however, the 50% of normal power used by analog filter410 and A/D converter 420 may or may not still be considered too high.

Returning to FIG. 3, interface circuitry 316 may provide an interfacefor exchanging audio and control data with telephone device 330. Inother words, interface circuitry 316 may allow CM 310 to function as anMTA. Interface circuitry 316 may be configured to route data from Rx 314to telephone device 330, and may also route data from telephone device330 to Tx 312. In one implementation consistent with the principles ofthe invention, interface circuitry 316 may control the communication ofcertain types of data, such as a sleep mode notification message sent toCMTS 140. When interface circuitry 316, or another controller in CM/MTA310, determines that low power mode should be initiated, interfacecircuitry 316 may direct Tx 312 to send such a sleep mode notificationmessage.

In one implementation consistent with the principles of the invention,wake-up circuitry 318 may be configured to receive a wake-up messagefrom CMTS 140 on downstream channel 220 during a low power mode ofCM/MTA 310. Wake-up circuitry 318 may be configured to receive aspecifically-modulated signal, such as the IP or hardware address ofCM/MTA 310, which may be amplitude modulated (AM), frequency modulated(FM), time division multiplexed, or code division multiplexed. Wake-upcircuitry 318 may be configured to recognize its address in the receivedsignal, and to wake up (i.e., resume power to) other elements in CM 310,such as Tx 312, Rx 314, and interface circuitry 316.

Wake-up circuitry 318 may be designed to consume lower power than theanalog filter 410 and A/D converter 420 in Rx 314. Because wake-upcircuitry 318 may monitor downstream channel 220 during CM/MTA 310's lowpower mode, power may be removed from all components 410-450 (i.e.,analog and digital) of Rx 314. Wake-up circuitry 318 (e.g., a low powerFM receiver) need only be able to reliably receive a wake-up signal(e.g., CM/MTA 310's address or other unique identifier) and wake upother components of CM/MTA 310. Subject to these performanceconstraints, those skilled in the communication art will understand thatwake-up circuitry 318 make include a number of circuit designs that mayconsume relatively little power when operating.

Telephone device 330 may be configured to generate VoIP data from inputaudio data, and to decode received audio data into a sound signal for auser. Telephone device 330 may include, for example, a handset, adialing apparatus (e.g., a key pad), and circuitry to generate anddecode VoIP data. Those skilled in the telephony art will understand thestructure and operation of typical telephone device 330. In otherimplementations, telephone device 330 may be a conventional telephonefor which CM/MTA 310 handles any VoIP to audio conversions.

CMTS 140 may include signaling circuitry 350 corresponding to thewake-up circuitry 318 in CM/MTA 310 (and possibly CM 320). Signalingcircuitry 350 may be configured to send the wake-up message to bring CM310/320 out of low power mode when, for example, an incoming telephonecall or other event (e.g., a timer timing out due to synchronizationloss with CMTS, or user making a phone call) dictates that CM 310/320 bereturned to normal operation. Signaling circuitry 350 may be configuredto generate a CM-specific wake-up message, such as a message addressedto CM310 (or CM 310's address itself), and may also be configured toinsert this wake-up message in normal downstream data communications onchannel 220. Signaling circuitry 350 may be configured to encode thewake-up message in a format that the wake-up circuitry may decode, suchas an AM signal, an FM signal, a time division multiplexed signal, or acode division multiplexed signal.

In one implementation consistent with the principles of the invention,signaling circuitry 350 and wake-up circuitry 318 may use a dedicateddownstream channel to communicate, which is different than the normaldownstream channel 220 used by CM 310 for downstream data communication.In such an implementation, wake-up messages from CMTS 140 would notinterfere with downstream data on channel 220 that is intended for otherCMs that are not in a low power mode, such as CM 320. By using adifferent downstream channel for wake-up messages to CMs in a low powerstate, CMTS 140 would not affect the operation of other CMs (e.g., 320)that are not in low power mode. In such an implementation, however, thetotal downstream bandwidth available to CMTS 140 is lowered, because thededicated downstream channel(s) used for wake-up messages may not beused for ordinary downstream data communication.

In another implementation consistent with the principles of theinvention, signaling circuitry 350 and wake-up circuitry 318 may beconfigured to use the normal downstream data channel 220 for wake-upmessages. Downstream data chunks from CMTS 140 are typicallyinterleaved, and also typically have associated error correction schemes(e.g., checksums), as understood by those skilled in the cable modemart. For these and other reasons, downstream data chunks may have anerror rate (e.g., 1 in 10⁸ bits) that is better than a nominal errorrate (e.g., 1 in 10⁶ bits) that is correctable at CMs 310/320 by theassociated error correction schemes. As a result, discrete units of timemay exist within a chunk or chunks of data which may be used to transmitother data than the chunks. Any other data inserted at these times willbe “corrected” by the error correction scheme at Rx 314 of receiving CMsand/or MTAs, but this other data may still be useful to wake upcircuitry 318.

Accordingly, signaling circuitry 350 may insert relatively small wake-upmessages in the downstream data on channel 220, without causing theerror rate (e.g., 8 in 10⁷ bits) to be worse than the nominal errorcorrection rate. Wake-up circuitry 318 in any low-powered CMs (e.g.,310) may recognize the wake-up messages and rouse Tx 312 and Rx 314 ifappropriate. Other normally operating CMs (e.g., 320) will see thesewake-up messages as “noise” in their data, and the error correctionschemes associated with the downstream data chunks will prevent thewake-up messages from affecting the operation of these CMs.

Exemplary Process

FIG. 5 illustrates an exemplary process for entering and exiting a CMpower saving mode consistent with the principles of the invention. Theprocess may begin with CM/MTA 310 making a decision to enter a low-powermode [act 510]. The decision by CM/MTA 310 may be influenced by a localpower outage, or it may be the result of some continuous period (e.g.,30-60 minutes) of inactivity. CM/MTA 310 may send a sleep modenotification message to CMTS 140 on upstream channel 210 via Tx 312 [act515].

If CMTS 140 allows CM/MTA 310 to enter a low power mode, it may stopand/or reset short term timers that would otherwise flag/remove CM/MTA310 after some period of inactivity [act 525]. Longer-term timers maystill require CM/MTA 310 to respond after some period of inactivity.CMTS 140 may allow CM/MTA 310 to enter the low power mode if no messagesare incoming for CM/MTA 310. CMTS 140 may also send an acknowledgment(Ack) message to CM/MTA 310 on downstream channel 220 [act 520]. The Ackmessage conveys to CM/MTA 310 that it has approval to enter therequested low power mode.

Upon receiving the Ack message, CM/MTA 310 (e.g., wake-up circuitry 318or a controller in CM/MTA 310) may power down certain circuitry (e.g.,Tx 312, Rx 314, etc.) to cause CM/MTA 310 to enter the low power mode[act 530]. Powering down such circuitry may include disconnecting orswitching a voltage supply from such circuitry. After or concurrent withentering the low power mode, wake-up circuitry 318 may begin listeningfor a wake-up message from CMTS 310, such as the address of CM/MTA 310[act 535]. In one implementation consistent with the principles of theinvention, act 535 may include providing power to wake-up circuitry 318,which may not receive power or operate when CM/MTA 310 is not in the lowpower mode.

When in low-power mode CM/MTA 310 may listen to a separate downstreamchannel or from time to time activate the downstream receiver 314 or usethe low-power receiver 312 to ensure that the synchronization betweenCMTS 140 and the CM/MTA 310 is not lost.

At some point after sending the Ack signal, CMTS 140 may determine, viaa wake up event occurring, that CM/MTA 310 needs to exit low power mode[act 540]. Such a wake up event may be the arrival of a telephone callfor CM/MTA 310. Another wake up event may be the expiration of a longterm timer in CMTS 140, at which point it may be beneficial for CMTS 140to check on the operation of CM/MTA 310 by causing it to exit low powermode. Other wake up events are possible, and will be apparent to oneskilled in the telecommunication art. CMTS 140 may encode a wake-upsignal into the existing DOCSIS downstream data on channel 220 [act545].

Wake-up circuitry 318 may receive and recognize the wake-up signal fromCMTS 140 [act 550]. In one implementation, recognizing the wake-upsignal includes detecting the address of CM/MTA 310 within data indownstream channel 220. The format of the wake-up signal (e.g., an FMsignal) may differ from that of the other data in downstream channel220. Other types and formats of wake-up signals are possible. Wake-upcircuitry 318 or another controller in CM/MTA 310 may provide power tothe circuitry (e.g., Tx 312, Rx, 314) from which power was removed toenter low power mode [act 555].

CM/MTA 310 may send (via Rx 314) an “awake” message to CMTS 140indicating that it has returned to normal-power operation [act 560].CMTS 140 may acknowledge the awake message with a normal DOCSIS messageon downstream channel 220 [act 565]. The Ack message may be received byRx 314, and CM/MTA 310 may resume normal (i.e., non-low power) operation[act 570]. For example, CM/MTA 310 may query CMTS 140 to see if thereare any pending messages or telephone calls that may have triggered itsexit from the low power mode.

In one implementation, after the ‘sleeping’ message [act 515] fromCM/MTA 310, CMTS 140 may stop and reset timers that might otherwisecause CM/MTA 310 to be removed from the list of active CM/MTAs 310 dueto its inactivity in accordance with DOCSIS specifications. One exampleof such timer-driven events is Station Maintenance messages from CMTS140. For example, CMTS 140 may send a Station Maintenance message to CM310 every 2 seconds, and after 16 or so re-tries, CMTS 140 may decidethat CM 310 is no longer alive and may remove it from its list of activeCM/MTAs 310. CMTS may, at the same time, drop all the packets destinedfor the now-inactive CM/MTA 310.

In contrast to such an example, and consistent with the principles ofthe invention, by receiving the ‘sleeping’ message [act 515] CMTS 140may stop and reset certain timers [act 525] so that it will not sendStation Maintenance messages. Hence, CMTS 140 will not remove the CM 310for failure to respond. Although such a scheme may not allow CM/MTA 310to power off Tx/Rx circuitry 312/314, it may advantageously allowpowering down of the processor (not shown) in CM/MTA 310 that processesDOCSIS MAC management messages and performs other functions. Normally,such processing would consumes a considerable power in CM/MTA 310. Eventhough the power savings to CM/MTA 310 in such an implementation may beless than in other schemes (e.g., turning off Tx 312, Rx 314, etc. asdescribed above in act 530), this implementation does not necessitatehardware upgrade/change to CM/MTA 310.

CONCLUSION

Consistent with the principles of the present invention, a low powermode for components in CMs may be utilized, while maintaining amechanism for an associated CMTS to wake up a CM that is in the lowpower mode. The mechanism may include circuitry in the CM other than thereceiver to cause components of the CM to exit the low power mode.

The foregoing description of embodiments of the present inventionprovides illustration and description, but is not intended to beexhaustive or to limit the invention to the precise form disclosed.Modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention.

For example, it should be noted that CMs which just handle data may alsocontain wake up circuitry 318 so that such CMs may enter a low powermode. Further, multiple CMs or MTAs that are serviced by the same CMTSmay enter and exit the low power mode, as long as there are enoughdistinct wake up signals/messages available so that the CMTS may wakeone CM/MTA without waking others.

While a series of acts has been described in FIG. 5, the order of theacts may vary in other implementations consistent with the presentinvention. Also, non-dependent acts may be performed in parallel.

No element, act, or instruction used in the description of the presentapplication should be construed as critical or essential to theinvention unless explicitly described as such. Also, as used herein, thearticle “a” is intended to include one or more items. Where only oneitem is intended, the term “one” or similar language is used. The scopeof the invention is defined by the claims and their equivalents.

What is claimed is:
 1. A method performed by a device, the methodcomprising: receiving a sleep message from a cable modem indicating adecision of the cable modem to enter a low power state; and stopping, inresponse to the sleep message, one or more timers that control removalof inactive cable modems from an active list due to inactivity of theinactive cable modems.
 2. The method of claim 1, where the stoppingincludes resetting the one or more timers.
 3. The method of claim 1,where one of the one or more timers includes a timer associated with aStation Maintenance message.
 4. The method of claim 1, where the one ormore timers include one or more short-term timers.
 5. A systemcomprising: a device to: receive a sleep message from a cable modemindicating a decision of the cable modem to enter a low power state; andstop, in response to the sleep message, one or more timers that controlremoval of inactive cable modems from an active list due to inactivityof the inactive cable modems.
 6. The system of claim 5, where, whenstopping the one or more timers, the device is to reset the one or moretimers.
 7. The system of claim 5, where one of the one or more timersincludes a timer associated with a Station Maintenance message.
 8. Thesystem of claim 5, where the one or more timers include one or moreshort-term timers.
 9. A device comprising: circuitry to: receive a sleepmessage from a cable modem indicating a decision of the cable modem toenter a low power state; and stop, in response to the sleep message, oneor more timers that control removal of inactive cable modems from anactive list due to inactivity of the inactive cable modems.
 10. Thedevice of claim 9, where, when stopping the one or more timers, thecircuitry is to reset the one or more timers.
 11. The device of claim 9,where one of the one or more timers includes a timer associated with aStation Maintenance message.
 12. The device of claim 9, where the one ormore timers include one or more short-term timers.